Nanotipped device and method

ABSTRACT

A dispensing device has a cantilever comprising a plurality of thin films arranged relative to one another to define a microchannel in the cantilever and to define at least portions of a dispensing microtip proximate an end of the cantilever and communicated to the microchannel to receive material therefrom. The microchannel is communicated to a reservoir that supplies material to the microchannel. One or more reservoir-fed cantilevers may be formed on a semiconductor chip substrate. A sealing layer preferably is disposed on one of the first and second thin films and overlies outermost edges of the first and second thin films to seal the outermost edges against material leakage. Each cantilever includes an actuator, such as for example a piezoelectric actuator, to impart bending motion thereto. The microtip includes a pointed pyramidal or conical shaped microtip body and an annular shell spaced about the pointed microtip body to define a material-dispensing annulus thereabout. The working microtip may be used to dispense material onto a substrate, to probe a surface in scanning probe microscopy, to apply an electrical stimulus or record an electrical response on a surface in the presence of a local environment created around the tip by the material dispensed from the tip or to achieve other functions.

This application is a division of U.S. Ser. No. 10/801,928 filed Mar.16, 2004 now U.S. Pat. No. 7,250,139, which claims priority and benefitsof provisional application No. 60/455,898 filed Mar. 19, 2003.

This application claims the benefits and priority of provisionalapplication Ser. No. 60/455,898 filed Mar. 19, 2003.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was supported by funding under National ScienceFoundation Grant/Contract No. EEC-0118025 and CMS-0120866. TheGovernment may have certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a dispensing device having one or morecantilevers each with a dispensing microtip that is supplied withmaterial via a microchannel in the cantilever and to microtips for otheruses, as well as to methods of dispensing material using such devicesand to methods of making such devices.

BACKGROUND OF THE INVENTION

Dip-pen nanolithography (DPN) has been developed to dispense material bymolecular transport from a cantilever tip mounted on the probe of anatomic force microscope (AFM) onto a substrate surface as described inScience 283, 661 (1999). DPN is disadvantageous in that repeatedre-inking of the dispensing tip is required to replenish molecules whenlarge surface areas are to be patterned or when complex patterns arerequired. DPN also suffers from low speed and throughput due to itsserial nature and limited scan size.

Another technique for nanopatterning involves a micropipette disposed onthe tip of an AFM probe as described in Appl/Phys. Lett. 65 (5), 648(1994). The micropipette technique suffers from the disadvantage ofirregular shape of the micropipette, low reproducibility, and lowresolution. The micropipette is difficult to integrate in array formatto carry out massive pattering operations on one or more substrates.

Still another technique referred to as millipede:parallel read/write isdescribed in IEEE Trans. on Nanotech. 1 (1), 39 (2002) and involves acantilevered AFM probe having a heated tip able to write on athermoplastic substrate by embossing the tip into the thermoplasticmaterial. The technique does not dispense any material, but isillustrative for a massively parallel writing method using AFM probearrays.

SUMMARY OF THE INVENTION

The present invention provides in an embodiment a dispensing devicehaving a cantilever comprising a plurality of thin films arrangedrelative to one another to define a microchannel in the cantilever. Amaterial dispensing microtip is disposed proximate an end of thecantilever and is communicated to the microchannel to receive materialtherefrom. In a particular embodiment, the microchannel is communicatedto a reservoir that supplies material to the microchannel. One or morereservoir-fed cantilevers may be formed on a semiconductor chipsubstrate. A sealing layer preferably is disposed on one of a pair ofthin films and overlies outermost edges of the thin films to seal anygap at the outermost edges against material leakage. The outermost edgesof the pair of thin films may include angled regions extending fromrespective planar film regions and wherein the sealing layer residesbetween the thin films at the angled regions. The dispensing device mayinclude an actuator, such as for example a piezoelectric actuator, oneach cantilever to impart bending motion thereto.

In another embodiment of the invention, a device is provided having aworking microtip with a pointed microtip body and an annular shellspaced about the pointed microtip body to define an annular spacethereabout. The pointed tip body may be formed by the substrate or by afirst thin film with the shell formed by another thin film. In aparticular embodiment for dispensing material, the pointed microtip bodycomprises a material more hydrophilic than the material defining theshell. For example, a first thin film or the substrate of hydrophilicmaterial can define the pointed microtip body and another thin film ofless hydrophilic nature can define the shell. For purposes ofillustration and not limitation, the working microtip may be used todispense material onto a substrate, to probe a surface in scanning probemicroscopy, to apply or record an electrical signal on a surface or toachieve other functions.

In still another embodiment of the invention, a method is provided formaking a device of the types described above by forming on a substrate aplurality of thin films arranged relative to one another to define anelongated cantilever precursor having a microchannel extending at leastpartly along the length of the cantilever precursor, by forming amicrotip proximate an end of the cantilever precursor and communicatedto the microchannel, and by releasing a portion of the cantileverprecursor from the substrate to form a cantilever extending from thesubstrate such that the cantilever has the microchannel and workingmicrotip thereon. In a particular method embodiment, amaterial-containing reservoir is provided on a semiconductor chipsubstrate and one or more cantilevers extend(s) from the chip substrate.

In a further embodiment of the invention, a method is provided formaking a microchannel by depositing first, second and third thin filmson a substrate, removing an outermost edge region of the second thinfilm to form an open-sided microchannel between the first and third thinfilms, and sealing the outermost edge region of the open side of themicrochannel by depositing a fourth thin film on one of the first andthird thin films so as to overlie the outermost edge region of the openside.

In still a further embodiment of the invention, a method of making aworking microtip involves forming a pointed tip on a substrate,depositing a plurality of thin films on the pointed tip, and removingregions of certain thin films about the pointed tip to form a materialdispensing annular space about the pointed tip. In an additionalembodiment of the invention, the fabrication of a sharper workingmicrotip involves the formation of a pointed tip on a substrate, localion implantation of the pointed tip with an etch stop, deposition of thefirst layer on the substrate, local removal of the first layer aroundthe tip region to expose the ion implanted tip region, deposition ofsecond and third thin films on the substrate, and removal of regions ofthe second and third thin films about the ion implanted pointed tipregion to form an annular space disposed between the pointed tip and thethird film and extending about the pointed tip.

In an even further embodiment of the invention, a method is provided toimprove communication between the material dispensing annular space andthe microchannel in the event the sealing layer is deposited by a CVDmethod on internal surfaces of the microchannel. The method involves,before applying the sealing layer, removing additional material from thesecond thin film concurrently and/or subsequently with removal of theoutermost edges of the second thin film of the cantilever until thematerial dispensing annular space is formed about the pointed tip andcommunicates with the microchannel.

A method of nanopatterning pursuant to an embodiment of the inventioninvolves supplying writing material through a microchannel in acantilever extending from a substrate to a material dispensing microtipproximate an end of the cantilever and moving the dispensing microtipclose enough to a surface to dispense the writing material thereon bydiffusion-driven molecular transport from the microtip. A particularembodiment supplies writing material from a microreservoir on thesubstrate to the microchannel and then to the dispensing microtip bycapillary action and/or diffusion. The writing material is dispensedthrough an annular space formed between a pointed microtip and anannular shell spaced about the microtip. The cantilever is moved duringnanopatterning by imparting a bend to it, such as by energizing athermal actuator or a piezoelectric film on the cantilever.

In practice of a further embodiment of the invention, a plurality ofworking microtips and cantilevers are integrated into linear or twodimensional arrays or stacks of two dimensional arrays to carry outparallel writing wherein each cantilever is controlled independently by,for example, addressing and actuating an actuator on each cantilever.

Further details and advantages of the present invention will becomeapparent from the following detailed description taken with thefollowing drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an atomic force microscope (AFM) headhaving a holder on which an illustrative writing material dispensingdevice shown schematically mounted thereon for writing on a surface.

FIG. 2 is a schematic perspective view of a dispensing device pursuantto an illustrative embodiment of the invention.

FIG. 3A is a longitudinal sectional view of a cantilever extending froma semiconductor chip substrate.

FIG. 3B is a partial sectional view of the dispensing microtip on thecantilever.

FIG. 4A shows a layout of an illustrative dispensing device for asemiconductor chip substrate. FIG. 4B shows the layout enlarged in thevicinity of the cantilevers.

FIG. 5 is an enlarged view of the device layout of FIG. 4A.

FIG. 6 is an enlarged view of the device layout of FIG. 4B.

FIGS. 7A, 7B, 7C illustrate a method of fabrication of the workingmicrotip. FIG. 7D is a photomicrograph of a microtip made by the methodof fabrication.

FIGS. 8A through 8I illustrate a microfabrication method according to anembodiment of the invention to make the cantilevers with microchannelsand working microtips. FIGS. 8A through 8D and FIG. 8I relate tofabrication of features shown in FIG. 5 along line A-A′. FIGS. 8E-8Irelate to fabrication of features shown in FIG. 6 along line B-B′. FIGS.8J and 8K are photomicrographs of actual cantilevers and a microtipprior to release from the silicon chip substrate. FIG. 8L shows abackside release mask.

FIGS. 9A, 9B, 9C illustrate a method of fabrication of themicrochannels.

FIG. 10 is a photomicrograph of an end portion of a cantilever made bythe microfabrication method.

FIGS. 11A, 11B, 11C, 11D, 11E illustrate another method of fabricationof the microtip.

FIGS. 12A and 12B show two different configurations of the fluid-airinterface achievable at the microtip of FIG. 11D.

FIGS. 13A, 13B, 13C, 13D illustrate sealing of the longitudinal edge orside of a microchannel wherein the sealing layer is deposited in themicrochannel. FIG. 13E is a photomicrograph of a transversely sectionedcantilever sealed by a sealing layer that extends inside themicrochannel pursuant to FIG. 13D. FIG. 13F is a photomicrograph of thedispensing end of a cantilever sealed by a sealing layer. FIG. 13G is aphotomicrograph of the microtip along the dotted lines of FIG. 13F.

FIGS. 14A, 14B, 14C illustrate another alternative embodiment forsealing of the longitudinal edge or side of a microchannel whereindeposition of the sealing layer on surfaces in the microchannel isreduced. FIG. 14D is a photomicrograph of a transversely sectionedcantilever sealed by a sealing layer that extends partially inside themicrochannel.

FIGS. 15A through 15N, 15P through 15Q, 15S through 15V, and 15X through15Y illustrate another method of fabrication of an embodiment of theinvention to make the cantilevers with microchannels and ultrasharpmaterial-dispensing microtips.

FIG. 16A shows a layout of another illustrative dispensing device for asemiconductor substrate chip wherein piezoelectric actuators are to beformed on the cantilevers during device fabrication. FIG. 16B shows thelayout enlarged in the vicinity of the cantilevers showing apiezoelectric film actuator.

FIG. 17 is an exploded schematic view showing fabrication of a massivelyparallel array of writing probes using the actuated cantileversdescribed above pursuant to the invention for high speed direct writingover a large area. FIG. 17A is a schematic perspective view of theunderside of a cantilever showing the integrated piezoelectric actuatorand cantilever with microchannels and dispensing tip.

DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an atomic force microscope (AFM) scanning head 10having a tip carrier 12 is illustrated schematically for purposes ofillustration and not limitation. A dispensing device 100 pursuant to anillustrative embodiment of the invention is schematically shown disposedor mounted on the tip carrier 12 for movement (e.g. raster scanning)with the scanning head 10 to dispense writing material M on a surface S.More particularly, a silicon semiconductor chip having the dispensingdevice 100 fabricated thereon is mounted on the tip carrier 12 in thesame manner as a conventional AFM probe tip such that the AFM hardwareand software can be used to move the dispensing device to write apattern with nanometer resolution of pattern features on surface S.

Referring to FIGS. 2, 3A, and 3B, dispensing device 100 pursuant to anillustrative embodiment of the invention is shown schematically ascomprising a semiconductor chip substrate C on which the dispensingdevice is fabricated by micromachining techniques as described below.The dispensing device 100 comprises one or more elongated cantilevers102 that each comprise a plurality of thin films arranged relative toone another as described below to define an elongated cantilever bodyhaving a microchannel 104 therein and to define a material-dispensingworking microtip 106 proximate an end of each cantilever remote from thechip substrate C. Each microtip 106 is communicated a respectivemicrochannel 104 to receive material M therefrom to be dispensed fromthe microtip onto the surface S. In turn, each microchannel 104 iscommunicated to a common material-containing reservoir 108 that suppliesmaterial M to the microchannel, although each microchannel may becommunicated to its own respective material-containing reservoir. InFIG. 3B, the fluid dispensing microtip 106 is shown comprising a pointedcore tip body 107 having a radius R and an annular, generally truncatedconical converging shell 109 spaced about the core tip to define amaterial dispensing annular space or annulus 110 residing about the coretip. The shell 109 converges in a direction toward the apex of the coretip body 107. To provide control of equilibrium of the fluid-airinterface at the annulus 110, the core tip body 107 preferably compriseshydrophilic material (e.g. silicon nitride, silicon oxide, metals) andthe shell 109 preferably comprises an equal or less hydrophilic material(e.g. silicon nitride, silicon, doped silicon).

Referring to FIGS. 4A, 4B, 5 and 6 which represent a device layout to befabricated on the chip C for a particular exemplary embodiment of theinvention, the microchannels 104 are shown having first and secondside-by-side channel regions 112 a, 112 b separated by a wall 113 withthe channel regions 104 a, 104 b terminating in a common arcuate channelregion 104 c extending partially about the core tip body 107 to supplymaterial thereto from the reservoir 108. In FIGS. 4A, 4B, 5 and 6, fivecantilevers 102 of different lengths (e.g. from 300 microns to 500microns with cantilever stiffness of 0.05 to 0.4 N/m) were formedextending from the chip C to evaluate cantilever length effects. Themicrochannels in the cantilevers 102 had a width of 4 to 7 microns whilethe tip 107 had a height of 3-5 microns relative to the surface of theadjacent cantilever 102 for demonstration purposes.

In the embodiments of invention for dispensing material, the material Mmay comprise a writing fluid (designated as “ink”) such as analkanethiol liquid solution (e.g. saturated solution of1-octadecanethiol in acetonitrile) onto surface S, which may comprisegold, for purposes of illustration and not limitation to form ananopattern. The alkanethiol molecules are transported by diffusion andcapillary action from the one or more microtips 106 to the gold surfaceand have a chemical affinity for the gold surface to attach thereto bychemisorption to form a monolayer. However, the invention is not limitedto any particular writing fluid (liquid) or other material to bedispensed from microtips 106. For example, for purposes of illustrationand not limitation, the material may comprise any chemical molecule,biomolecule (e.g. DNA, protein, etc.), or other species. The molecule orspecies may or may not be in a liquid aqueous or organic solution ordispersed in a liquid carrier. Moreover, a solid material may bedispensed from the dispensing device 100 by surface diffusion or acombination of surface diffusion and capillary action intermediated orfacilitated by a meniscus formed by capillary condensation between thetip and the substrate of the moisture present in the ambient or adsorbedonto the substrate. For example, a writing material dispersed, dissolvedor otherwise present in a fluid carrier is supplied from the reservoirthrough the microchannel to the microtip where the writing material mayor may not solidify or transform to a solid or be present as a solidmaterial sans the fluid carrier (e.g. the carrier fluid dries or isotherwise removed) at the microtip. Dispensing of the writing materialfrom the microtip to the surface S can be facilitated by formation of ameniscus out of water present on the surface S and/or in the ambientenvironment or atmosphere. For purposes of illustration and notlimitation, a material including, but not limited to, 1-octadecanethiol,may be dispensed from the microtips 106 by the combineddiffusion/capillary condensation action mechanism.

The material, whether a fluid or a solid, can be deposited on surface Sto form a pattern with nanometer resolution of pattern features, toinitiate local reactions, to effect exchange of ions, to performvoltametry in nanometer-size confined spaces and for other purposes.

Potential applications for the dispensing device 100 include, but arenot limited to, DNA nanopatterning involving depositing DNA forsequencing and/or synthesis, protheomics, combinatorial nanochemistry,nanolithography involving dispensing photoresist or other resistmaterials, scanning probe microelectrochemistry involving imaging,etching, deposition, and nanovoltametry, scanning probe chemistryinvolving etching, deposition, and mask repair, and nanojets and atomguns involving localized delivery of free radicals and atom species.

Furthermore, in practice of the invention, the cantilevers 102 withmicrotips 106 are not limited to applications where a material isdispensed from the microtips. For example, a working microtip 106 may beused to probe a surface in scanning probe microscopy, to apply or recordan electrical signal on a surface, or to achieve other functions withoutdispensing any material therefrom.

The microfabrication process of the dispensing device 100 uses thedevice layouts shown in FIGS. 4A, 4B, 5 and 6 and begins with a {100}single crystal silicon chip substrate.

Referring to FIGS. 7A, 7B, 7C, each core tip body 107 is first formed onthe chip substrate C. For example, for each core tip body 107, a mask120 is placed on the chip substrate C, and the chip is etched using oneor more of various conventional etchants to form the pyramidal-shaped(or other shape) core tip body 107. A suitable etching treatmentcomprises a KOH etching in 40% (mass ratio) KOH and HNA etching inhydrofluoric-nitric-acetic acid mixture (3:5:3 by volume), althoughother etchants can be used including, but not limited to, XeF₂, SF₆reactive ion etching, and the like. The pointed tip body 107 issharpened by oxidizing the etched chip substrate C to thermally grow aSiO₂ layer thereon, FIG. 7B, followed by wet etching of the oxidizedchip substrate using buffered hydrofluoric acid process. The core tipbodies 107 are formed concurrently on the chip substrate C in this way.A plurality of individual pointed, pyramidal-shaped or conical-shapedtip bodies 107 having tips 107 t thereby are formed on the chipsubstrate, FIGS. 7C and 7D.

FIGS. 8A through 8I illustrate a microfabrication method for forming acantilever 102 on the chip substrate C (having the core tip bodies 107)pursuant to a method embodiment of the invention. Each cantilever 102 iscommunicated to reservoir 108 via microchannel 104, FIGS. 8A through 8I.All of the cantilevers 102 are formed concurrently on the chip substrateusing the method illustrated wherein FIGS. 8A through 8D and 8I relateto fabrication of features shown in FIG. 5 along lines A-A′ and FIGS.8E-8H relate to fabrication of features shown in FIG. 6 along linesB-B′.

Referring to FIG. 8A, the chip substrate is shown with a core tip body107 formed as described above. In FIG. 8B, a first thin film or layer130 of Si₃N₄ (e.g. 0.25 microns thick) is deposited by LPCVD (lowpressure chemical vapor deposition) on the chip substrate. Then, asecond thin film or layer 132 of SiO₂ (e.g. 0.5 microns thick) isdeposited by LPCVD on the first thin film 130. Then, a third thin filmor layer 134 of Si₃N₄ (e.g. 0.3 microns thick) is deposited by LPCVD onthe second thin film 132. A photoresist layer 136 then is applied onthin film 134, FIG. 8B.

Referring to FIG. 8C, the thin film coated chip C is patterned withphotoresist layer 136 and then etched using CF₄ reactive ion etching(RIE) for etching the third layer (nitride) and using bufferedhydrofluoric acid solution to selectively remove a portion of the SiO₂film or layer 132 as shown to produce the undercuts 138, 140, 141 whichwill form portions of the microchannel in the cantilever. FIG. 8D takenalong line A-A′ and 8E taken along line B-B′ show enclosure of the openedges of the undercuts 138, 140, 141 by selective oxidation of thesilicon chip substrate by thermal wet oxidation process to produce a“bird's beak”.

FIG. 9A illustrates partial etching of the SiO₂ layer 132 that occurs toform open sided microchannel regions 104 a, 104 b separated by and onopposite sides of wall 113 on each cantilever 102 concurrently withformation of the undercuts 138, 140, 141 of FIG. 8C.

FIG. 9B shows the effect of selective oxidation of the chip substrate Cat areas OX to cause the first film or layer 130 to bend toward thethird film or layer 134 to form an outermost angled region 130 a at anoutmost edge thereof concurrently with the selective oxidation of FIG.8D. The angled region 130 a extends from a generally planar region 130 bof the first film or layer 130.

Then, the selective oxidation step is followed by deposition of asealing layer 142 by sputtering, or evaporation or CVD process asillustrated in FIG. 8D along lines B-B′ and in FIG. 9C. The sealinglayer 142 can comprise Si₃N₄ or polycrystalline silicon (Poly-Si)sealing material, or any other suitable sealing material. The sealinglayer 142 overlies the outermost edges of the first and third films asshown best FIG. 9C to prevent leakage of fluid or material from themicrochannel regions 104 a, 104 b. FIG. 10 is a photomicrograph of acantilever 102 after the sealing layer 142 is deposited thereon asillustrated in FIGS. 9A through 9C, which are taken along lines A-A′ ofFIG. 10.

Referring to FIGS. 8F through FIG. 8I, photoresist 144 is deposited inthe desired pattern on the sealing layer 142, FIG. 8F. The photoresist144 is partially removed by oxygen RIE (reactive ion etching) to exposethe apex or end of the microtip 106, FIG. 8G.

Then the exposed apex of the microtip, starting with the sealing layerand continued with the third layer are etched by CF₄ RIE, FIG. 8G. Theremaining photoresist is removed by oxygen plasma. The selective removalby buffered hydrofluoric solution of the second thin film or layer 132of SiO₂ from between the first and third films or layers 130, 134 at thecore tip body 107, FIG. 8H, occurs until the annular space or annulus110 so formed communicates with the microchannel. This completes theformation the volcano-shaped microtip 106, FIG. 8I which is taken alonglines A′A′ of FIG. 5. The microtip 106 thereby is formed to includepointed core tip body 107 and an annular, generally conical convergingshell 109 (comprised of the third film or layer 134) spaced about thecore tip body to define a material dispensing annular space or annulus110 residing about the core tip body. The sealing layer 142 resides onthe conical converging shell 109. FIGS. 8J and 8K are photomicrographsof cantilevers and a microtip prior to release from the chip substrate.

Then, a KOH or other etching step is conducted to release thecantilevers 102 from the Si substrate. In this final etching step, thereservoir 108 also is formed in the chip substrate so as to be in fluidflow communication with the microchannels 104 on the cantilevers 102.The dispensing device 100 eventually is released from the siliconsubstrate by the KOH etching from the backside of the chip substratewith a suitable backside release mask designated shown in FIG. 8Lpresent on the backside. The backside release mask eventually containsconvex corner compensation beams such as described in Sensors andActuators, Vol. 3, p. 127, 1992, incorporated herein by reference. KOHetching involves wet etching of the chip substrate using 40% (massratio) KOH at 80 degrees C. Subsequently, the side of the chip opposingthe tip will be coated by evaporation with a reflective metal film, suchas Au (gold) 15 nm, to provide better reflection of the laser beameventually used for the AFM head positioning control (optical lever).The dispensing device 100 of FIG. 3A is thereby microfabricated on thesilicon substrate chip.

For purposes of illustration and not limitation, the microfabricationmethod described above can be used to produce individual cantilevers 102having a length of about 100 microns to about 500 microns and includinga microchannel 102 having a width dimension in the range of about 4 toabout 10 microns and a height dimension in the range of about 0.05 toabout 1.5 microns. Flow rate of fluid through a microchannel is affectedby channel dimensions, fluid wetting properties of the microtip andmaterials of the microtip and microchannel, the chemical pretreatment,and fluid viscosity. Similarly, the core tip body 107 can be produced tohave an apex having a height of about 3 to about 5 microns of the tiprelative to a plane of the cantilever (0.5 to 1.5 microns above theplane defined by the end of the shell 109). The inner radius of the endof the shell 109 measured from the apex of the core tip body 107 can bein the range of 0.5 to 2 microns.

FIGS. 11A through 11D illustrate an alternative embodiment for formingthe microtip wherein like reference numerals are used to designate likefeatures of previous figures. In FIG. 11A, the core top body 107 isillustrated as having been doped with boron (B) which doped region BRfunctions as an etch stop during later KOH etching step for the releaseof the cantilever illustrated in FIG. 11E, to form the volcano-shapedmicrotip 106 differing from that of FIGS. 3A and 8I in having a pointedend 107 t of the boron doped core tip body 107 protruding above thefirst thin film or layer 130 of Si₃N₄ as shown in FIGS. 11D and 11E.Such a microtip of FIG. 11D-11E permits two possible configurations forthe fluid-air interface at the microtip 106 as illustrated in FIGS. 12A,12B, respectively. A pressure pulse of about 1 atmosphere on the fluidis needed to switch from the fluid-air interface of FIG. 12A to that ofFIG. 12B. An alternative way to bring the fluid interface to that of 12Bis to dip the dispensing tip into liquid.

FIGS. 13A, 13B, 13C illustrate an alternative embodiment for sealing ofthe longitudinal edge or side E of a microchannel regions 104 a, 104 bdefined between the first and third thin films or layers 130, 134 ofSi₃N₄ and separated by wall 113 formed by partially etching the secondthin film or layer 132 of SiO₂. In this alternative embodiment, thesealing layer 142 is deposited in the microchannel regions 104 a, 104 b.For example, referring to FIG. 13B, during the selective oxidation stepwhere the chip substrate C is oxidized at areas OX (SiO₂), both thefirst and third thin films or layers 130, 134 have been observed to bendslightly away from the chip substrate so as to have angled region s 130a, 134 a at an outermost edges thereof. A small gap is formed betweenthe angled regions 130 a, 134 a, as illustrated in FIG. 13B. Subsequentdeposition of the sealing layer 142 by LPCVD has been found to result indeposition of the sealing layer 142 on the surfaces inside themicrochannel regions 104 a, 104 b as illustrated in FIG. 13C (beforeetching of the substrate chip) and FIG. 13D (after etching of the chipsubstrate) to effect sealing of the microchannel regions. The internaldeposition of the sealing layer on the inner walls of the microchannelsdoes not reduce substantially the cross section and the fluid transportin the channels, since there is a fluid transport (flow) only during thefilling of the microchannel, not during the writing process, whichrelies mostly on the diffusion of ink molecular species along thechannels, rather than a flow of the fluid ink itself. However, thedeposition of the sealing layer on the inner sidewall of themicrochannels may hinder the establishing of the connectivity betweenthe microchannel and the shell-to-core gap 110 of the dispensing tip,during etching step illustrated in FIG. 8I. FIG. 13E is photomicrographof showing the sealing layer deposited inside the microchannel regionsof an actual microfabricated cantilever that has been transverselysectioned by a focused ion beam. FIG. 13F is photomicrograph of showingthe sealed microchannel regions of an actual microfabricated microtipthat has been transversely sectioned through the microtip. FIG. 13G isphotomicrograph of showing the sealing layer deposited inside themicrochannel regions around the core tip of an actual microfabricatedmicrotip that has been transversely sectioned by focused ion beam alongthe dotted line of FIG. 13F. Thus, the fabrication method described byFIG. 8A-8I typically is used only for sealing layers deposited by lowconformity processes, such as evaporation and sputtering.

FIGS. 14A, 14B, 14C illustrate still another alternative embodiment forsealing of the longitudinal edge or side of a microchannel regions 104a, 104 b defined between the first and third thin films or layers 130,134 of Si₃N₄ by wall 113 formed by partially etching the second thinfilm or layer 132 of SiO₂. This embodiment addresses the case thesealing is performed by depositing the sealing layer by a highconformity process, such as CVD. In this alternative embodiment, thefirst thin film or layer 130 comprises a low stress SiN layer and thethird thin film or layer 134 is modified to comprise a dual layerstructure comprising a low stress SiN layer 134 s and high stress SiNlayer 134 t that bends in response to residual internal stress towardthe first thin layer or film 130 to, FIG. 14A, reduce the size of thegap at the outermost edges of these films or layers. A low stress SiNfilm or layer is deposited by LPCVD (low stress nitride process, basedon higher Si content in the film and higher deposition temperatures,such as 875 degrees C), while a high stress SiN is deposited by standardLPCVD stoichiometric nitride deposition process.

FIG. 14B shows the effect of selective oxidation of the chip substrate Cto cause the first film or layer 130 to bend toward the third film orlayer 134 to form an outermost angled region 130 a at an outmost edgethereof concurrently with the selective oxidation. A low stress SiNsealing layer 142 then is deposited on the third thin film or layer 134to seal the smaller gap with reduced penetration of the sealing layerinto the microchannel regions 104 a, 104 b, FIG. 14C. FIG. 14D isphotomicrograph of showing the sealing layer deposited partially insidethe microchannel regions of an actual microfabricated cantilever thathas been transversely sectioned.

FIG. 15A through 15Y (where like features of previous figures aredesignated by like reference numerals) illustrate an embodiment of theinvention in which a sharper tip is microfabricated on a cantilever 102and the connectivity (communication) of the microchannels 104 to theshell-to-core annular space 110 of the dispensing tip is increased, towork with a subsequent high conformity sealing layer deposition, leadingto improved sealing of the microchannels.

FIGS. 15A through 15B illustrate a microfabrication method for forming asharp tip body 107 by depositing a masking layer 200, patterning it bylithographic means and etching the silicon substrate isotropically,followed by oxidation sharpening (this process reproduces the previousprocess described in FIG. 8A). FIG. 15C illustrates the lithographicremoval of the oxide 202 around the tip body 107 by wet etching, such asusing buffered hydrofluoric acid and the ion implantation of an etchstop dopant, such as boron 5×10¹⁹ B/cm³, to form an implanted tip bodyregion BR followed by the removal of the oxide by buffered hydrofluoricacid, FIG. 15D. Then, a first thin film layer 130 such as low stresssilicon nitride of thickness from 0.25 μm to 1 μm and a temporarymasking layer 206 such as oxide of thickness from 0.25 μm to 0.5 μm aredeposited by LPCVD methods. A lithographic mask 208 is applied to definean exposed region around the tip body 107 that is 1-4 μm. less in radialsize than the ion implanted region BR and also defining simultaneouslythe shape of the future reservoir, FIG. 15E. The temporary masking layer206, such as oxide, is wet etched using buffered hydrofluoric acid inthe openings of the photoresist mask, FIG. 15F. The photoresist isremoved, and the first layer 130, such as nitride, is wet etched toexpose the pointed tip 107 t on the substrate C, such as using hotphosphoric acid in the openings provided by the temporary masking layer206, FIG. 15G. Then, the temporary masking layer 206, such as oxide, isremoved by buffered hydrofluoric acid, FIG. 15H. Then, a second thinfilm layer 132 such as low temperature silicon oxide and a third thinfilm layer 134, such as low stress silicon nitride are deposited on thesubstrate by LPCVD, FIG. 15I and patterned lithographically to definethe shape of the cantilever precursor and future channels, FIG. 15J. Thethird thin film layer 134 can eventually be a sandwich of high stressnitride and low stress nitride, such as in the process described in FIG.13A-13D, to provide later a better sealing by bending the third thinfilm layer 134 towards the first thin film layer 130. The third, secondand first thin film layers are etched through the lithographic mask 210,using reactive ion etching, such as with CF₄ gas, FIG. 15K. Then, aselective wet chemical etching of the second thin film layer 132 isperformed, such as by using buffered hydrofluoric acid, to defineconcurrently the open-side microchannels 104 and shell-to-tip annularspace 110 of the dispensing microtip 106, FIG. 15L. The etching of thesecond thin film layer 132 at this step is to be performed until thesecond thin film layer is etched around the apex of the tip body 107. Ifthe desired width of the microchannels 104 is etched before the secondthin film layer is etched away around the apex of the tip body 107, anadditional lithographic mask 214 can be applied and the second thin filmlayer etching can be subsequently continued locally, only around the tipbody 107, as illustrated in FIG. 15M. After the etching of the secondthin film layer 132 is conducted to form the desired size ofmicrochannels 104 and shell-to-core annular space 110 at the dispensingtip, a thermal oxidation of the silicon chip C at area OX for example isperformed, FIG. 15N, to provide an angling of the first thin film layer130 so as to close the gap or space at the outermost edges of themicrochannel 104, through a process known as “birds beak oxidation” forthose skilled in the field and/or as described above with respect toFIGS. 9A-9C, 13A-13D, and 14A-14D. Subsequently, a sealing layer 142,including but not limited, to a low stress silicon nitride is depositedby any deposition method, including but not limited to LPCVD. In case ofa deposition method of high conformity, such as LPCVD, the sealing layer142 may cover the inner surfaces of the microchannels 104 and theannular space 110 around the tip body 107, without affecting theirfuture fluid communication, FIG. 15P. The device microfabricationcontinues by defining the future cantilever shape lithographically, FIG.15Q, via patterning with photoresist 216 of the sealing layer 142, usinga selective etching such as like CF₄ RIE in case the sealing layer issilicon nitride. The same photoresist layer or another photoresist layercan be used to first decover the tip apex 107 t by thinning thephotoresist with oxygen plasma etching, then to continue with reactiveion etching (such as CF₄ RIE) to etch the sealing layer 142 and thethird thin film layer 134 around the tip apex 107 t, FIGS. 15S and 15Twhich also present the schematic evolution of the tip shape during theetching process. After the complete removal of the photoresist by usingoxygen plasma or/and commercial photoresist remover, the devicestructure appears as shown in FIG. 15T. Then, the backside alignment andlithography are performed, eventually using a mask containing convexcorner compensation beams, such as the one in FIG. 8L. The backsidemasking layer corresponds to the first, second, third and sealing thinfilm layers, which can be removed sequentially using reactive ionetching such as CF₄ RIE. or combinations of RIE and wet chemicaletching, FIG. 15U. Then, a backside silicon etching of the silicon chipsubstrate C is performed using a front-side protecting holder and 40%(mass ratio) KOH solution at 80 degrees C, or any other method ofetching of the silicon chip substrate while protecting the structures onthe front side of the chip substrate, FIG. 15V. The backside siliconremoval step also defines the reservoir 108 and the chip substrate Ceventually mechanically attached to a larger silicon frame for easyhandling, while leaving the cantilever structure embedded in a compositemembrane composed of oxide, the first, second, third and the sealingthin film layers, out of which the oxide and the second thin film layerare subsequently wet-etched, such as using buffered hydrofluoric acid,FIG. 15X. If necessary, the backside of the chip substrate can befurther etched using CF₄ RIE to connect the microchannels 104 to thereservoir 108 by removing the sealing layer 142 eventually covering theinner side surfaces of the microchannels. A very thin metal layer, suchas but not limited to Au 15 nm, can be deposited on the backside of thechip substrate C in order to increase the reflection of the cantilever102 for the laser beam eventually used for the control of the AFM probepositioning (optical lever). Use of the fabricated device and microtipto write or probe is achieved with the tip oriented towards the writingor probing surface S as illustrated in FIG. 15Y.

In an embodiment where the device is used as a probe to apply or recordelectrical signals through the tip 107 t, the metal used for enhancingthe laser reflection properties of the backside of the cantilever 102can be also used to apply these signals to surface S, since it connectsthe body of the chip substrate C with the eventually boron doped silicontip body 107, which is conductive.

FIGS. 16A, 16B and 17, 17A illustrate an embodiment of the inventionwherein an actuator 150 is disposed on each cantilever 102 to impartbending motion thereto to move the microtip 106 close enough to thesurface S to effect dispense writing material thereon, or in embodimentwhere no material is dispensed, close enough to probe surface S or toapply or record an electrical signal on surface S. The actuator 150 maybe selected from any suitable actuator such as including, but notlimited to, a piezoelectric actuator having a piezoelectric film,thermal actuator having a resistor forming a composite with the rest ofthe cantilever, with different thermal expansion coefficients, or amagnetic actuator having a magnetic film and others.

Referring to FIGS. 16A, 16B, a piezoelectric actuator 150 is illustratedas being formed on the surface of the cantilever 102 on which themicrotip 106 is disposed. The piezoelectric actuator 150 is formed bydepositing on that surface a Pt film 152 sandwiched between the Tifilms, a piezoelectric (PZT) film 154, and a Au (gold) film 158separated by a Ti film from the PZT film. The Pt film 152 provides anelectrical contact by which the PZT film 154 is connected by electricallead 160 to ground, and the Au film 158 provides an electrical contactby which the PZT film 154 is connected by electrical lead 162 to asource of electrical voltage or current (not shown) to energize thepiezoelectric film 154 in a manner to cause the cantilever 102 to bendtoward or away from the surface S. Each actuator 150 thereby can beaddressed and actuated independently by a suitable electronic controllersuch as a microcomputer (not shown) to independently actuate thecantilevers 102 to move during operation of the dispensing device 100.The first-deposited Pt film can be used eventually also as a reflectivelayer for enhancing the reflection properties of the cantilever foroptical position control within the AFM equipment (optical lever).

FIG. 17 illustrates schematically fabrication of a dispensing device 200having a plurality of cantilevers 102 of the type described aboveintegrated in linear and two dimensional arrays as shown for purposes ofillustration and not limitation. The device 200 is shown including onecommon reservoir 108 to supply material to all of the cantilevers 102but more than one reservoir 108 can be provided as desired. For example,a reservoir 108′ could be provided for each row of cantilevers. Eachcantilever 102 includes a PZT actuator 150 that is addressed andactuated independently by a suitable controller (not shown). FIG. 17Ashows a cantilever 102 with the actuator 150 thereon. Those skilled inthe art will appreciate that one or more other devices 200 can bestacked atop and/or below the device 200 shown to provide stacks of twodimensional cantilevers 102 arrays in a manner to form a threedimensional cantilever array where the cantilevers are independentlyaddressed and actuated by respective integral actuators 150 on thecantilevers 102 in response to a multiplexed addressing scheme. Sucharrays can be used to produce massively parallel active cantilevers formaterial (e.g. ink, biomolecules, etc.) dispensing applications withcontinuous-material delivery or feed to material dispensing microtips106 for high speed direct writing over large surface areas.

For purposes of illustration and not limitation, a dispensing device ofthe type shown in FIG. 8I was mounted for testing on the tip holder of aconventional atomic force microscope Dimension 3100, Digital Instrumentshaving a closed loop scanner to minimize thermal drift during writing ofa pattern on surface. Writing using the dispensing device was testedusing 1-octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHA) ona Au (gold) surface. After the reservoir 108 was filled with ODT or MHA,the microtip 106 was brought into contact with a solution of ODT or MHAspread on the gold surface for tip-priming purposes. As a writing test,letters with a minimum line width of about 200 nm were successfullyobtained by dispensing the ODT or MHA onto the gold surface; then theletters were imaged using the same microtips as for dispensing, inlateral force imaging AFM mode. Imaging in constant force mode was alsotested on a calibration standard (Pacific Nanotechnology), consisting ofan array of squares, with different lateral sizes in the 1-10 μm range,and heights of 80 nm. The imaging capabilities of the novel microtips ofthe invention in the tapping and contact modes are completely similar tostandard AFM probes (VEECO, NP-20. Likewise, they exceed theirresolution and sensitivity in lateral force mode as a result of thelower rotational stiffness in twisting of the cantilevers.

The invention envisions a method of applying an electrical stimulus andmeasuring the electrical response of a surface in nanometer-scalevicinity of a probing microtip 106 in the presence of a locally createdenvironment at the end of the microtip through which the material isdispensed around the microtip. For example, an electrolyte material,such as including but not limited to, HCl, NaCl, copper sulfate, and thelike, can be dispensed from the volcano-shaped microtip 106 of thecantilever 102 onto the surface to create the local environment. Theelectrical stimulus can be a constant or varying voltage or electricalcurrent applied by the microtip 106 to the surface by appropriatemovement of the cantilever 102 to bring the microtip close enough to orin contact with the electrolyte and/or the surface to apply theelectrical stimulus thereto or to record an electrical response at agiven location on the surface. The method can be used to characterizethe dispensed material or the surface at the given location.

The invention is advantageous in providing a material dispensing devicethat can provide continuous feeding of material to one or more microtipsand the possibility to fabricate arrays of material dispensing deviceseasy to integrate on microfluidic chips and capable of parallel writingwith one or several material species.

Although the invention has been described in connection with certainembodiments thereof, those skilled in the art will appreciate that themodifications and changes can be made thereto without departing form thespirit and scope of the invention as set forth in the appended claims.

1. A device, comprising a tip having a pointed tip body and an annularshell comprising an etched thin film spaced about the tip body to definean annular empty space about the tip body wherein a semiconductor chipsubstrate or another thin film defines the tip body.
 2. The device ofclaim 1 wherein the tip body comprises a material more hydrophilic thanthe shell material.
 3. The device of claim 1 wherein said substrate orsaid another thin film defines the tip body and the etched thin filmdefines the shell.
 4. The device of claim 1 that includes a cantileverconnected to the tip body wherein the cantilever includes a microchannelcommunicated to the annular empty space.
 5. A device, comprising a tiphaving a pointed tip body and an annular shell comprising a thin filmspaced about the tip body to define an annular space about the tip bodywherein the tip body comprises a material more hydrophilic than that ofthe thin film.